Such a pressure control system is described in WO-A-2005/082744, wherein the high-pressure vessel is substantially cylindrical with a tapered neck portion and a flange, on which a ring-shaped insert or closure with a step-like funnel is mounted. The pressure control device of this document is provided within a ring-shaped closure by a cup-like cylinder having a closed end and an open end, in which a piston with a downward protruding stem and a broader cylindrical end portion is movable mounted. The piston is sealed by an O-ring to the inner wall of the cup-like cylinder and can be moved in a reciprocating manner. At the open end of the step-like funnel an O-ring is pinched by a ring-cylinder, whereas the broader cylindrical portion of the stem and the O-ring provide a valve which is moved in a reciprocating manner by the pressure of a pressurizing gas, i.e. air, in the chamber formed by the piston and the cup-like cylinder with its closed end.
When mounting the piston into the cup-like cylinder, it is very important that the outer wall of the O-ring will not be impaired or damaged. Already very small incisions will influence the life-span of the pressure control device, since pressurized gas or air could escape and diminish the reference pressure gradually. A similar problem occurs with the pinched O-ring, which provides a valve with the broader cylindrical portion of the stem. Although in use the piston and the stem will have a reciprocating movement of less than one mm, there will be little wear, which influences the longevity of the pressure control system.